Mammalian cells respond to a variety of extracellular signals by undergoing proliferation, differentiation or apoptosis. These events are stringently regulated by a complex machinery comprising of signaling molecules, tumor suppressor proteins, transcription factors and their downstream targets. This proposal will focus on the role of Prohibitin, a potential tumor suppressor protein, in the above processes. Prohibitin gene is located on chromosome 17q21 and its mutations have been found in sporadic breast cancers. Our earlier studies had shown that prohibitin could bind to the retinoblastoma tumor suppressor protein and repress the transcriptional activity of E2F family members. This was necessary for prohibitin to inhibit cell proliferation. Our recent studies show that prohibitin could enhance the transcriptional activity of p53. Based on these findings, we will examine whether the transcriptional co-repressors HP1 and SUV39H contribute to prohibitin-mediated repression of E2F1 activity and whether prohibitin contributes to heterochromatin formation during cellular senescence. Attempts will be made to understand how prohibitin regulates the YY1 promoter and how this contributes to regulation of cell proliferation. We will next examine the mechanisms by which prohibitin facilitates the recruitment of p53 to promoters, leading to its activation. We will focus on whether prohibitin influences the tetramerization of p53 as well as its post-translational modifications. The last specific aim will focus on in vivo models to study prohibitin function. We have generated transgenic mice over-expressing prohibitin in mammary epithelial cells. We will examine the effects of prohibitin on mammary gland development as well as carcinogen and Ras induced breast carcinoma. Experiments are in progress to generate a conditional prohibitin knock-out mice. We expect that these experiments will help elucidate the molecular mechanisms involved in prohibitin-mediated growth control and tumor suppression.